Against a background of intensifying movements for environmental protection, regulations on emissions of carbon dioxide and similar gases have been tightened. In the automobile world, development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) is being vigorously pursued in addition to vehicles using gasoline, diesel oil, natural gas and other fossil fuels. Furthermore, the soaring rise in the price of fossil fuels over recent years has given a boost to the development of EVs and HEVs.
For such EVs and HEVs, nickel-hydrogen secondary batteries and lithium ion secondary batteries are commonly used. Vehicles are now required not only to be environmentally friendly, but also to have high-level basic performance, that is, driving ability as automobiles. In order to raise the driving capability to high levels, it is necessary not merely to make the battery capacity larger but to make the battery output larger, since this has a major effect on the automobile's acceleration performance and climbing performance. However, when high-output discharge is performed, large current flows in the battery, and as a result, the heat generated by the contacting resistance between the battery's substrate and current collecting body becomes large. Thus, batteries for EVs and HEVs are required not only to be large-size and large-capacity, but also to be able to produce large current. Consequently, in order to prevent power loss inside the batteries and reduce heat generation, various improvements have been implemented concerning preventing poor welding between the battery's substrate and current collecting body, and thus lowering the internal resistance (refer to JP-10-261441-A (claims, paragraphs 0011 to 0014, FIGS. 8 to 10) and JP-2000-200594-A (paragraphs 0002 to 0007, 0018 to 0021, FIGS. 1, 2, 3, 8, 9)).
Generally, nickel-hydrogen secondary batteries and lithium ion secondary batteries for EVs and HEVs have a rolled electrode in an elongated cylindrical shape composed of strips of positive and negative electrodes rolled with a strip separator interposed therebetween. Taking the lithium ion secondary battery as an example, negative electrode active materials such as graphite coated to a surface excluding an upper end of a thin strip of copper foil, which is a negative electrode substrate, is used for a negative electrode, positive active materials such as lithium cobalt complex oxide coated to a surface excluding a lower end of a thin strip of aluminum foil, which is a positive electrode substrate, is used for a positive electrode. These negative and positive electrodes form a rolled electrode by shifting up and down a little while rolling, so that an upper end exposing the negative electrode substrate is protruded from above, and a lower end exposing the positive electrode substrate is protruded from below.
A current collecting body welded to negative and positive electrode substrates at an uncoated side is formed with a slit. Each edge of the substrates at the uncoated side is inserted into the slit and this area is irradiated by a laser, thereby laser-welding each edge of the substrates at the uncoated side and the current collecting body (refer to JP-10-261441-A and JP-2000-200594-A). This method enables reliable welding of an edge of the substrates at an uncoated side of an electrode and a current collecting body, thereby reducing internal resistance of a battery, and enabling a battery to be obtained with reduced variation of resistance.
The connection between an edge of substrates at an uncoated side of the electrode and a current collecting body disclosed in JP-10-261441-A will now be described with reference to FIGS. 7 and 8. FIG. 7 is a perspective view of a current collecting body used for a battery disclosed in JP-10-261441-A. FIG. 8 is a schematic plan view of laser welding after mounting a current collecting body to an edge of substrates at an uncoated side to a current collecting body of FIG. 7. This current collecting body 40 is made of the same material as the substrates of an electrode plate, having a thickness of 5 mm, and a gap between slits 41 of 0.2 mm. At a surface side, an edge of the current collecting body protrudes by 0.5 mm from the outer surface of each slit 41. An undersurface side is provided with an insertion groove 42 which is in communication with the slit 41 at which a plurality of substrate edges are inserted bundled together.
The negative electrode side of the current collecting body 40 with such features, for example, as shown in FIG. 8, is mounted to a rolled electrode 45 so as an edge 44 of a plurality of substrates is protruded from the slit 41 of a current collecting body 401 at a negative electrode 43 side. Along the edge 44 of the substrates protruded from the slit 41 of the current collecting body 401, the outer surface of the slit 41 section of the current collecting body 401 and the edge 44 of the substrates are laser welded by tilting a light axis of a laser beam 46 by θ=15° from the welding surface.
Next, the connection between substrate edges at an uncoated side of an electrode and the current collecting body disclosed in JP-2000-200594-A will be described with reference to FIGS. 9 through 12. FIG. 9 is a perspective view showing the state in which a current collecting body is mounted to a rolled electrode of a battery disclosed in JP-2000-200594-A. FIG. 10 is a partially enlarged perspective view of the current collecting body of FIG. 9. FIG. 11 is an enlarged sectional view before the substrate edges are bent at an uncoated side of an electrode inserted into a slit of the current collecting body of FIG. 9. FIG. 12 is an enlarged sectional view after the substrate edges are bent at an uncoated side of an electrode inserted into a slit of the current collecting body of FIG. 9.
A rolled electrode body 50 of a battery disclosed in JP-2000-200594-A is connected with a current collecting body 52 of a negative electrode 51 and a current collecting body (not shown) of a positive electrode 53. The current collecting body 52 of the negative electrode 51 is a copper alloy plate covering above a straight section in a central portion and one side of a curved section of the rolled electrode body 50 in an elongated cylindrical shape. A pinching section 54 is provided at the area above one side of the straight section, created by folding the copper alloy plate to have an inverted U-shaped profile. Each pinching section 54 has a window section 55 making a gap by grinding a part of the folded copper alloy plate except for the both ends of its top end. Further, at the area covering above the curved section of the elongated cylindrical shape of the rolled electrode 50, a lower end of a negative electrode terminal 56 made of a copper alloy is connected. A current collecting body of the positive electrode 53 is made of an aluminum alloy plate (not shown) covering above, the sides, and lower side of the straight section in the center, and above the other curved section of the elongated cylindrical shape of the rolled electrode 50. At the tip of the straight section on the lower side, a pinching section is provided that is formed in the same manner as the pinching section 54 of the current collecting body 52 of the negative electrode 51, created by folding the aluminum alloy plate to have an inverted U-shaped profile.
The window section 55 of the pinching section 54 of the battery disclosed in JP-2000-200594-A is deeper than the related art, and is grinded by leaving a pressing plate piece 58 protruded upward from one of the opposite sides of the copper alloy plate of the pinching sections 54. Further, the pressing plate piece 58 shown in FIG. 12 is in a squashed state (this process is described later). The pressing plate piece 58 has a thinner plate thickness than one of the opposite sides of the copper alloy plate of the pinching sections 54, and is grinded into a plate-form at a width to have a gap at both ends of the window section 55. In the gap of the pinching section 54 of the negative electrode current collecting body 52 stated above, as shown in FIG. 11, a plurality of substrates at an upper end of the negative electrode 51 of the rolled electrode 50 are pinched together, so as a tip is fully protruded to the deeply grinded window section 55. In this way, the tip of copper foil of the negative electrode 51 protruded to the window section 55, as shown in FIG. 12, is curved towards an opposing direction from the side that the pressing plate piece 58 is protruded, by pressing the pressing plate piece 58 from obliquely above, so that the substrates of the negative electrode 51 curved at the end of the pressing plate piece 58 is fixed so as to be pressed down. Accordingly, the tip of the substrates of the negative electrode 51 which is pressed down at the tip of the pressing plate piece 58 is connected to the surrounding copper alloy plate by laser welding. The pinching section of a positive electrode current collecting body of a battery disclosed in JP-2000-200594-A has similar features to the current collecting body of the negative electrode.
According to the features disclosed in JP-2000-200594-A, for example, the substrates at an upper end of the negative electrode 51 protruded from the window section 55 of the pinching section 54 of the negative electrode current collecting body 52 have an end that is fixed in a bent manner, making it difficult for these substrates to come loose from the pinching section 54, even if the substrate tip is not properly welded or even if disconnected. Further, an excessive force will not be applied onto the welded tip of these substrates, as the substrates at the upper end of the negative electrode 51 are firmly pinched to the pinching section 54 as above, making it difficult for the weld to disconnect. Therefore, even if the battery is mounted on an electric vehicle and the like and receives vibration and impact repeatedly, the connection between the negative electrode 51 and the negative electrode current collecting body 52, and the positive electrode 53 and the positive electrode current collecting body of the rolled electrode 50 is not at risk of easy disconnection, thereby preventing deterioration of battery performance.
According to the examples of the related art as described above, the substrates of the negative and positive electrodes and the current collecting body are connected by laser welding, thereby reducing internal resistance of a battery, whereby a battery can be obtained with reduced variation of resistance. However, the structure with the slit 41 of the current collecting body 40 shown in FIGS. 7 and 8 of JP-10-261441-A includes substrates 44, for example, formed with a number of copper foils at an upper end of the negative electrode 43. As the upper end is only laser welded in a state that the substrates 44 are bundled and pinched in the slit 41 of the negative electrode current collecting body 40, thereby causing problems such as the laser welding becoming incomplete and easily disconnected. When the laser-welded area disconnects, the thin substrates 44 of the negative electrode 43 are pinched by the slit 44 overlapping with a number of other substrates 44 of the negative electrode 43. Accordingly, when subjected to vibration and impact, there is a possibility of some substrates coming loose. Also when even only one substrate 44 of the negative electrode 43 comes loose from the slit 41, the pinching force against the rest of the substrates 44 of the negative electrode 43 becomes weak, thereby causing a problem that these substrates 44 are also apt to come loose.
In the connection disclosed in JP-10-261441-A, a possibility of internal short-circuit arises because laser beam is irradiated from above the bundled substrates 44, whereby sputtered fine particles can infiltrate the rolled electrode during the irradiation of laser beam.
The pinching section 54 of the battery disclosed in JP-2000-200594-A is deeper than the related art, and grinded by leaving a pressing plate piece 58 protruded upward from one of the opposite sides of the copper alloy plate of the pinching section 54. Consequently, the substrates at an upper end of the negative electrode 51 of the rolled electrode 50 are gathered in plurality and pinched so as the tip is fully protruded to the window section 55 deeply grinded in the pinching section 54. However, the length of the substrates at the upper end of the negative electrode 51 is generally assumed to be fixed, thereby causing a difference in distance to the tip of the substrates of the negative electrode 51 pinched within the pinching section 54, corresponding to the thickness of the rolled electrode 50. As a result, they will not be in a uniform height as shown in FIG. 11, but as shown in FIG. 13A, the center is the highest, and becomes shorter towards both ends in the pinching section 54.
Therefore, when the height L1 of the pinching section 54 is short, as shown in FIG. 13A, there is a possibility that some of the substrates at the upper end of the negative electrode 51 do not reach the window section 55. The problem may be solved, as shown in FIG. 13B, by raising the height L2 of the pinching section 54, enlarging the partially grinded window section 55, and increasing substrate margins of the negative electrode 51 being exposed. But when such features are adopted, the space that the rolled electrode occupies within a battery package decreases, thereby causing a problem of reduced battery capacity.
The pinching section 54 of the electrode disclosed in JP-2000-200594-A, at least at both sides of the pressing plate piece 58, is in a state that the substrates at the upper end of the gathered negative electrode 51 are exposed upward from the window section 55 as is disclosed in JP-10-261441-A. Accordingly, when the substrates at the upper end of the negative electrode 51 and the pinching section 54 are laser-welded at an area that the pressing plate piece 58 of the pinching section 54 does not exist, a possibility of internal short-circuit by sputtered fine particles arises, as is disclosed in JP-10-261441-A.